Chap. I. over contains titanic acid. He considers the gas thus obtained to be a combination of fluoride of silicon and fluoride of titanium. But he could obtain no gaseous fluoride of titanium when the mixture was heated in a leaden apparatus.* Berzelius analyzed the fluotitaniate of potash, and obtained It would appear (if any confidence can be put in this analysis) that the acid consists of History. Perhaps the half atom may have been in the state of hydrofluoric acid, simply combined with the fluotitanic acid composed of one atom titanic acid and one atom fluoric acid. The greater number of the acids belonging to this class are still very imperfectly known. Considerable obscurity still hangs over the combinations of fluoric acid with other acids, which can only be cleared up by farther investigation. The subject is attended with peculiar difficulties, because all the experiments require to be made in platinum or leaden vessels, which prevents us from observing the phenomena so well as when vessels of glass can be employed. CLASS VI. CYANOGEN ACIDS. We are indebted to an accident for the original fact from which the whole of this important branch of chemistry derives its origin. About the year 1710 Diesbach, a manufacturer of colours in Berlin, wishing to prepare some lake by precipitating a decoc * Poggendorf's Annalen, vii. 320. tion of cochineal, alum, and green vitriol, with potash, borrowed some alkali for that purpose from Dippel. This chemist was the discoverer of a peculiar animal oil which goes by his name. He prepared it from blood; and the alkali with which he furnished Diesbach had been employed in the process. Instead of the red precipitate which he expected, a beautiful blue powder fell to the bottom. On mentioning the circumstance to Dippel, that chemist ascribed the formation of the powder to the action of his alkali on the alum and vitriol. It is not unlikely that he had calcined the potash together with a portion of blood. Be that as it may, he succeeded in discovering a method of procuring the blue powder at pleasure, and it was announced as a pigment in the Berlin Miscellanies for 1710. The preceding history, however, was only communicated to the public by Stahl 20 years after.* Class VI. Prussian blue, This powder was called Prussian blue; and the method of Formation of procuring it remained concealed, because it had become a lucrative article of commerce, till Dr. Woodward published a process in the Philosophical Transactions for 1724, which he had procured, as he informs us, from one of his friends in Germany. This method was as follows: Detonate together four ounces of nitre and as much tartar, in order to procure an extemporaneous alkali; then add four ounces of dried bullock's blood; mix the ingredients well together, and put them into a crucible covered with a lid, in which there is a small hole; calcine with a moderate fire till the blood emits no more smoke or flame capable of blackening any white body exposed to it; increase the fire towards the end, so that the whole matter contained in the crucible shall be moderately but sensibly red. In this state throw it into four pounds of water, and boil it for half an hour. Decant off this water, and continue to pour on more till it come off insipid. Add all these liquids together, and boil them down to four pounds. Dissolve an ounce of sulphate of iron in half a pound of water, and eight ounces of alum in four pounds of boiling water: mix all the three solutions together while boiling hot. An effervescence takes place, and a powder is precipitated of a green colour. Separate this precipitate by filtration, and pour muriatic acid upon it till it becomes of a beautiful blue; then wash it with water and dry it. t *Stahl's Experiments, Observat. Animadvers. ccc. numero. Chim. et Phys. p. 281. + Phil. Trans. xxxiii. 15. Chap. I. Experiments of Macquer. Different explanations were given of the nature of this precipitate by different chemists. Mr. Brown immediately repeated the process of Woodward, ascertained that other animal substances, as beef, may be substituted for blood; that the alum is useful only to dilute the colour: and that the blue pigment is produced by the action of the alkali (altered by blood) on the iron of the vitriol. He ascertained, too, that prussian blue is insoluble in muriatic acid, and that the green colour is owing to a mixture of prussian blue and oxide of iron, and that the muriatic acid developes the blue colour by dissolving the oxide of iron.* These facts were of considerable importance; but they threw no light upon the theory of the process. An explanation of this was first attempted by Geoffroy, who had ascertained that any animal body whatever might be substituted for blood. According to him, the blood communicates a portion of inflammable matter, or phlogiston, to the alkali, and this inflammable matter revives the iron of the vitriol and brings it to the metallic state. A greater quantity of blood, by increasing the inflammable matter, will enable the alkali to revive still more of the iron, and thus to strike a blue at once, instead of a green.t Though this explanation was approved of at the time by the best chemists, it was far from satisfactory. Macquer soon after proceeded, by way of experiment, and added a new step to the facts ascertained by Brown. That celebrated chemist ascertained the following facts: 1. When an alkali is added to a solution of iron in an acid, the iron is usually precipitated of a yellow colour, and soluble in acids; but if iron be precipitated from an acid by an alkali prepared by calcination with blood (which has been called a prussian alkali), it is of a green colour. 2. Acids dissolve only a part of this precipitate, and leave behind an insoluble powder which is of an intense blue colour. The green precipitate therefore is composed of two different substances, one of which is prussian blue. 3. The other is an oxide of iron; and the green colour is owing to the mixture of the two substances. 4. When heat is applied to this prussian blue, its blue colour is destroyed, and it becomes exactly similar to common oxide of iron. It is composed therefore of iron and some other substance, which heat has the property of driving off. 5. If it be * Phil. Trans. 1724, xxxiii. 17. + Mem. Par. 1725. boiled with a pure alkali, it loses its blue colour also, and at Class VI. the same time the alkali acquires the property of precipitating of a blue colour solutions of iron in acids, or it has become precisely the same with the prussian alkali. 6. Prussian blue, therefore, is composed of iron and something which a pure alkali can separate from it, something which has a greater affinity for alkali than for iron. 7. By boiling a quantity of alkali with prussian blue, it may be completely saturated with this something, which may be called colouring matter, and then possesses the properties of a neutral salt. 8. When iron dissolved in an acid is mixed with an alkali saturated with the colouring matter, a double decomposition takes place; the acid unites with the alkali, and the colouring matter with the iron, and forms prussian blue. 9. The reason that, in the common method of preparing prussian blue, a quantity of oxide is precipitated, is, that there is not a sufficient quantity of colouring matter (for the alkali is never saturated with it) to saturate all the iron displaced by the alkali; a part of it therefore is mixed with prussian blue. Muriatic acid dissolves this oxide, carries it off, and leaves the blue in a state of purity.-Such were the conclusions which Macquer drew from his experiments. The nature of the colouring matter, however, was still unknown. Macquer supposed it to be phlogiston. According to him, prussian blue is nothing else than iron supersaturated with phlogiston. This overdose protects the iron from acids, and prevents the magnet from acting on it. Heat drives off this dose, and leaves the prussian blue in the state of common iron. From this theory, which differed but little from that of Geoffroy, the alkali saturated with the colouring matter of prussian blue received the name of phlogisticated alkali. Macquer having observed that it did not act on alkaline and earthy solutions, while it precipitated all the metals, proposed it as an excellent test for detecting the presence of these last bodies. The subsequent experiments of chemists threw an air of suspicion on Macquer's theory. Baumé ascertained, that when prussian blue is distilled, it always yields a portion of animal oil; a product not very likely to appear if the powder contained nothing but phlogiston and iron. Deyeux and Parmentier, Bergman, Erxleben, Delius, and Scopoli, submitted prussian blue to distillation, and obtained a quantity of ammonia. Fontana ascertained that prussian blue detonated with nitre. * See Macquer's Dictionary, i. 177. + Baumé's Chemistry, ii. 601. Chap. I. Scheele, Landriani obtained, by distillation, a little acid liquid and oil, and a great quantity of azotic gas and carburetted hydrogen gas. These facts were still more inconsistent, if possible, with Macquer's theory. Morveau advanced another in 1772; namely, that the phlogisticated alkali, besides phlogiston, contained also an acid which acted the principal part in the phenomena produced.* Sage affirmed that the colouring matter in phlogisticated alkali was phosphoric acid; but this opinion was refuted by Lavoisier.+ Bergman also announced his suspicions that it was an acid, but an unknown one.‡ Such was the knowledge of chemists respecting the nature of this colouring matter, when Scheele all at once removed the veil, and explained its properties and composition. This he performed in two dissertations on prussian blue, published in the Stockholm Transactions for 1782 and 1783.§ He observed that the prussian alkali, after being exposed for some time to the air, lost the property of forming prussian blue; the colouring matter must therefore have left it. Researches of He put a small quantity of it into a large glass globe, corked it up, and kept it some time; but no change was produced either in the air or the prussian alkali. Something must therefore displace the colouring matter when the alkali is exposed to the open air, which is not present in a glass vessel. Was it carbonic acid gas? To ascertain this, he put a quantity of prussian alkali into a glass globe filled with that gas, and in 24 hours the alkali was incapable of producing prussian blue. It is therefore carbonic acid gas which displaces the colouring matter. He repeated this experiment with this difference, that he hung in the globe a bit of paper which had been previously dipped into a solution of sulphate of iron, and on which he had let fall two drops of an alkaline lixivium in order to precipitate the iron. This paper was taken out in two hours, and became covered with a fine blue on adding a little muriatic acid. Carbonic acid, then, has the property of separating the colouring matter from alkali without decomposing it. He found also that other acids produce the same effect. Hence he concluded that the colouring matter might be obtained in a separate state. Accordingly he made a great many attempts to procure it in that state, and at last hit upon the following method: * Digressions Academiques, p. 249. + Mem. Par. 1777, p. 77. § Scheele, ii. 141. |